High-power voltage-converter circuits are used in many applications. Known converter circuits have three voltage levels and are used for operating rotating electrical machines, such as synchronous and asynchronous machines, which rotating electrical machines can have three stator windings. In a known method for operating a rotating electrical machine, the machine is connected phase-by-phase to a converter circuit such as this, which has a DC voltage circuit, for switching m voltage levels, where m≧2. The DC voltage circuit in a converter circuit for switching, for example, three voltage levels is formed by a first capacitor and by a second capacitor connected in series with the first capacitor, with the DC voltage circuit furthermore having a first main connection to the first capacitor, a second main connection to the second capacitor and a sub-connection, which is formed by the two series-connected capacitors. The converter circuit for switching three voltage levels furthermore has power semiconductor switches which are connected in a known manner.
In this context, in a three-phase converter circuit for switching three voltage levels, the phases of the converter circuit are, for example, connected to the DC voltage circuit in accordance with a selected switching state combination of switching states of the power semiconductor switches in the converter circuit. In the case of a converter circuit for switching three voltage levels, the phases of the converter circuit are accordingly connected to the first main connection, to the second main connection or to the sub-connection in accordance with a selected switching state combination of switching states of the power semiconductor switches of the converter circuit.
In a state diagram, such as that shown in FIG. 2, these switching state combinations and their transitions are shown with respect to one another, with the “+” representing a connection of the corresponding phase to the first main connection, “−” representing a connection of the corresponding phase to the second main connection, and “0” representing a connection of the corresponding phase to the sub-connection.
Appropriate desired switching state combinations are selected, for example, in accordance with the known direct torque control (DTC), in which the instantaneous actual value of the torque of the rotating electrical machine, of the magnetic stator flux of the rotating electrical machine and of the potential at the sub-connection are first of all each compared with an associated, predetermined value range. The respectively predetermined value range is or can be time-variant, and can be governed by a superordinate control loop having reference values of the torque of the rotating electrical machine, of the magnetic stator flux of the rotating electrical machine and of the potential at the sub-connection. If an instantaneous actual value now exceeds its associated predetermined value range, then a switching state combination is selected from a table as a function of the previous selected switching state combination, such that the instantaneous value which results for this switching state combination could, if desired, once again be within the associated value range, although there is no guarantee of this. Furthermore, a switching state combination can be selected either with respect to the instantaneous actual value of the torque, of the magnetic stator flux or of the potential on exceeding the associated value range. There is no joint analysis of the instantaneous actual value of the torque, of the magnetic stator flux and of the potential.
In a method as described above for operating a rotating electrical machine by the known “direct torque control,” there are a plurality of transitions between the previous selected switching state combination and the instantaneously selected switching state combination, and these are represented by lines between the switching state combinations in FIG. 2. The switching state combinations and the transitions from one switching state combination to another can be stored fixed in the table, although all the combination options of switching state combinations as shown in FIG. 2 need not be stored in the table. Furthermore, in the case of “direct torque control”, one switching state combination can be selected as a function of the previous selected switching state combination together with the associated transitions, which is stored in the table and which once again returns the instantaneous value resulting from the selected switching state combination back within the associated value range. Switching state combinations which can alternatively be selected, for example, with possibly fewer transitions from the previous selected switching state combination, are not stored in the table. A plurality of transitions between switching state combinations generate a multiplicity of switching operations of the power semiconductor switches in the converter circuit, however, as a result of which the switching frequency of the power semiconductor switches rises. However, a high switching frequency such as this can result in heat losses (greater energy consumption) in the power semiconductor switches in the converter circuit, as a result of which the power semiconductor switches can age more quickly, may be damaged, or may even be destroyed.
In this context, EP 1 670 135 A1 specifies a method for operating a rotating electrical machine, by which the switching frequency of power semiconductor switches of a converter circuit which is connected phase-by-phase to the rotating electrical machine and which is used for switching m voltage levels can be reduced, where m≧2. According to the method, in a step (a), the phases of the converter circuit are connected to the DC voltage circuit in accordance with a selected switching state combination of switching states of power semiconductor switches in the converter circuit. This switching state combination is selected in the following further steps:    (b) starting with a start sampling time k for a selectable number N of sampling times:            determination of all the switching state combinations relating to each of the N sampling times, wherein N≧1,            (c) formation of switching state sequences for each specific switching state combination relating to the start sampling time k, wherein each switching state sequence is a sequence of specific switching state combinations of the N sampling times associated with the respective switching state combination relating to the start sampling time k,    (d) for each of the switching state sequences, calculation of a torque trajectory of the rotating electrical machine and of a magnetic stator flux trajectory of the rotating electrical machine from the calculated state value sets of the rotating electrical machine and of the converter circuit for the start sampling time k to the sampling time k+N,    (e) selection of a switching state sequence for which an associated torque trajectory and a magnetic stator flux trajectory relating to the (k+N)-th sampling time are each within a predetermined value range, and setting this selected switching state sequence,    (f) repetition of steps (a) to (d), where k=k+1.
In the method for operating a rotating electrical machine according to EP 1 670 135 A1, method steps (b) to (e) can be carried out on a digital signal processor, in which case steps (b) to (e) are then, for example, in the form of a computer program which can be loaded. The multiplicity of calculation steps in the method according to EP 1 670 135 A1 involves computation power for a digital signal processor, thus resulting in very long computation times in the digital signal processor, and therefore also long execution times for the method steps, which can then result in the phases of the converter circuit being connected at the wrong time to the DC voltage circuit according to the selected switching state combination of switching states of the power semiconductor switches.
Furthermore, in the case of the method according to EP 1 670 135 A1, it is possible for the torque trajectory or the magnetic stator flux trajectory of each associated switching state combination relating to the k-th or to the (k+1)-th sampling time to already be outside the predetermined value range, in which case the method for operating a rotating electrical machine according to EP 1 670 135 A1 cannot handle a state such as this. However, only restricted operation of the rotating electrical machine is therefore possible.